Did the solar system form in a bubble?

Astronomers believe that planets, asteroids, and other solar system bodies form from the disk of dust and debris around a young star. But what happens before that?

NASA/JPL-Caltech

Astronomers know that our solar system formed about 5 billion years ago from material left over from previous generations of stars. However, beyond that, it gets a little murky.

The prevailing theory is that a nearby supernova explosion compressed a dense cloud of gas and dust until it collapsed in on itself due to its own gravity. As the cloud condensed, it grew hotter and spun faster. Eventually, the center of the cloud grew so hot it began fusing hydrogen into helium and became the star we lovingly call the Sun.

But according to a study published December 22 in the Astrophysical Journal, the solar system instead may have formed inside the dense shell of an enormous bubble within a giant star. The study not only provides a fantastical scenario for our solar system’s formation, but also addresses a long-standing mystery concerning our solar system’s chemical makeup.

The new theory for how the solar system formed starts with an extremely massive star known as a Wolf-Rayet star. Of all the stars in the universe, these stars burn the hottest. Because they are so hot, they also have exceptionally strong stellar winds.

As a Wolf-Rayet star sheds its outer layers – a normal end-of-life process for a giant star – its strong stellar winds plow through its loosely held cloak of material, forming densely shelled bubbles. According to the study, the solar system could have formed inside of one of these bubbles.

Since such a huge amount gas and dust is trapped inside, “the shell of such a bubble is a good place to produce stars,” said Nicolas Dauphas, co-author of the study and professor of geophysical sciences at the University of Chicago, in a press release. The researchers estimate that this stellar-womb process is so effective that it could account for the formation of 1 to 16 percent of all Sun-like stars.

This simulation shows how stellar winds can produce bubbles inside the loose envelop of material surrounding a giant star over the course of millions of years. Researchers from the University of Chicago have proposed the new theory that the solar system could have formed inside one of these bubbles.

V. Dwarkadas/D. Rosenberg

Although the unconventional theory may seem a bit superfluous, the researchers proposed it because it also addresses a long-standing mystery of the early solar system: Why did it have so much aluminium-26 and so little iron-60 when compared to the rest of the galaxy?

Previous studies of meteorite samples have shown that the early solar system was ripe with the isotope aluminium-26, while other studies have shown it was deficient in the isotope iron-60. However, since supernovae explosions produce both of these isotopes, “it begs the question of why one was injected into the solar system and the other was not,” said Vikram Dwarkadas, co-author of the study and professor of astronomy and astrophysics at the University of Chicago.

This is what brought the researchers to Wolf-Rayet stars, which produce lots of aluminium-26, but zero iron-60.

“The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star,” said Dwarkadas. “These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed – trapping the aluminum inside the shell.” Over time, the bubble stops pushing outward and falls back in on itself due to gravity. This collapsing bubble is where the researcher’s think our solar system could have formed.

Though the researcher’s new theory is far from accepted, its ability to explain the observed chemical composition of the solar system is sure to lead to future studies. In 2023, the NASA spacecraft OSIRIS-REx will return a sample of the ancient asteroid Bennu to Earth. Perhaps this will help astronomers unravel our solar system’s origin story?